Transparent Conducting Oxide Layer By Thermal Evaporation for Cu(In,Ga)Se₂ Solar Cell with Chemical Bath Deposited-ZnS Buffer Layer

Abstract

Chemical-bath-deposited (CBD) ZnS buffer layer is being steadily studied in Cu(In,Ga)Se2 (CIGS) thin-film solar cell because of their beneficial properties, for examples, good transparency with large direct bandgap, less toxicity and cost effectiveness. CBD-ZnS buffer layer, however, is easily affected during subsequent sputtering process for window layer and the conversion efficiency decreases with sputtering power. The decrease of efficiency in CBD-ZnS/CIGS thin-film solar cells is known to be related to plasma damage originated from high energy negative oxygen ions or neutral particles at the interface. In typical CIGS solar cell, ITO/i-ZnO bilayer deposited by sputtering process has been used as a conventional window layer and then interface properties between window layer and buffer is considered as an important key to determine the cell efficiency. In this paper, to know actual effects of interface property between window layer and buffer on cell performance, we applied the two types of window layer and fabricated CIGS solar cell with CBD-ZnS buffer; (i) typical structure of ITO/i-ZnO/CBD-ZnS/CIGS, and (ii) thermally grown-IZO/i-ZnO/CBD-ZnS/CIGS. To completely exclude plasma damage from sputtering, two types of CIGS solar cell were also fabricated without i-ZnO layer. As a result, the cell performance interestingly shows the opposite tendency depending on presence or absence of i-ZnO layer; cell efficiency with IZO layer is higher than that with ITO layer in the presence of i-ZnO layer, while the former is lower than the latter in the absence of i-ZnO layer. To understand these phenomena, interfacial properties between window and buffer layer are characterized by various analysis tools.